Osteosarcoma is one of the most frequently occurring malignant bone tumor except for multiple myeloma. However, osteosarcoma of the mandible is rare and aggressive malignancy constituting about 5% to 13% of all cases of skeletal osteosarcoma. The authors experienced a case of osteosarcoma of the mandible in a 31-years old male patient. He visited our outpatient clinic with painless mass of the cheek. On the basis of the clinical and imaging findings, the differential diagnosis included giant cell tumor, histiocytosis and malignant bone tumor. In the first operation, the mass of the mandible was excised. Intraoperative microscopic examination of a frozen section did not yield a diagnosis: however, the final definitive histologic examination revealed osteoblastic osteosarcoma. After 15days, wide excision with hemimandibulectomy was completed followed by postoperative radiotherapy and chemotherapy. With our experience, we report the progression of diagnosis and management for mandibular osteosrcoma.